By Daniel Muniz
The Open Systems Interconnection (OSI) and TCP/IP models are one of the first things you learn about in networking. They serve as a common point of reference when describing how data is transmitted over a network, how protocols relate to and inter-operate with one another.
But what does that really mean?
First, a little history.
The OSI model was originally conceived back in the 1970’s as a component of the OSI protocol suite, which was positioned as an early competitor to the emerging TCP/IP family of protocols. OSI protocols are not in common use today.
There are two key points to make about the OSI model. First, the OSI model is just that, a model. Its use is not mandated for networking, yet most protocols and systems adhere to it quite closely. It is mainly useful for discussing, describing, and understanding individual network functions.
Second, not all layers are used in some simpler applications. While layers 1, 2, and 3 are mandatory for any data transmission, the application may use some unique interface layer to the application instead of the usual upper layers of the model.
The OSI layers are as follows:
- Layer 7 – application: This layer works with the application software to provide communications functions as required. It verifies the availability of a communications partner and the resources to support any data transfer. It also works with end applications such as domain name service (DNS), file transfer protocol (FTP), hypertext transfer protocol (HTTP), Internet message access protocol (IMAP), post office protocol (POP), simple mail transfer protocol (SMTP), Telnet, and terminal emulation.
- Layer 6 – presentation: This layer checks the data to ensure that it is compatible with the communications resources. It ensures compatibility between the data formats at the applications level and the lower levels. It also handles any needed data formatting or code conversion, as well as data compression and encryption.
- Layer 5 – session: Layer 5 software handles authentication and authorization functions. It also manages the connection between the two communicating devices, establishing a connection, maintaining the connection, and ultimately terminating it. This layer verifies that the data is delivered as well.
- Layer 4 – transport: This layer provides quality of service (QoS) functions and ensures the complete delivery of the data. The integrity of the data is guaranteed at this layer via error correction and similar functions.
- Layer 3 – network: The network layer handles packet routing via logical addressing and switching functions.
- Layer 2 – data link: Layer 2 operations package and unpack the data in frames.
- Layer 1 – physical: This layer defines the logic levels, data rate, physical media, and data conversion functions that make up the bit stream of packets from one device to another.
TCP/IP was developed during the 1960’s as part of the Department of Defense’s (DoD) Advanced Research Projects Agency (ARPA) effort to build a nationwide packet data network.
There are actually two protocols that constitute this model TCP (Transmission Control Protocol) and IP (Internet Protocol). It was first used in UNIX-based computers in universities and government installations.
TCP/IP is also a layered protocol but does not use all of the OSI layers, though the layers are equivalent in operation and function.
Relationships and Differences:
- IP corresponds to a subset of OSI Layer 3, the network layer
- TCP corresponds to OSI Layer 4 (transport) and some functions of Layer 5 (session)
- TCP/IP makes no assumptions about what happens above the level of a network session (part of OSI Layer 5), while OSI defines several more layers of standardized functions
- TCP/IP makes no prescriptions as to the link layers below IP, where OSI specifies two.
- Where an application needs functions not found in TCP/IP, the application has to supply them. In the OSI model, it is assumed that an application will never implement any functionally belonging in any defined layer, and because interfaces between layers abstract many details, it may not be able to anyway.
- The OSI model is a “generic, protocol-independent standard”. TCP/IP is regarded as the standard around which the internet has developed.
- TCP/IP is a simple model because of fewer layers (four) as compare to OSI that defined seven layers in original version.
- TCP/IP is more reliable model as compare to OSI model because the internet was developed around it.
- The OSI model is bottom to up process of network connection whereas the TCP/IP is the top to bottom process structure for internet purpose.
- The TCP/IP combines the OSI data link and physical into network access layer.
- OSI model is a reference model while TCP/IP is an implementation of OSI model.
Technically speaking, no protocol from the TCP/IP stack has an official assignment to an OSI layer, because they’re not of the same family.
A reference model is just that, a reference. It helps illustrate the dependencies protocols have on another, and where they sit in relation to one another, but it doesn’t strictly govern their function.